Ultrasound is a valuable diagnostic imaging technique for studying various areas of the body including, for example, the vasculature, such as tissue microvasculature. Ultrasound provides certain advantages over other diagnostic techniques. For example, diagnostic techniques involving nuclear medicine and X-rays generally result in exposure of the patient to ionizing electron radiation. Such radiation can cause damage to subcellular material, including deoxyribonucleic acid (DNA), ribonucleic acid (RNA) and proteins. Ultrasound does not involve such potentially damaging radiation. In addition, ultrasound is relatively inexpensive as compared to other diagnostic techniques, such as magnetic resonance imaging (MRI), which can require elaborate and expensive equipment.
Ultrasound involves the exposure of a patient to sound waves. Generally, the sound waves dissipate due to absorption by body tissue, penetrate through the tissue, or reflect off the tissue. The reflection of sound waves off tissue, generally referred to as backscatter or reflectivity, forms the basis for developing an ultrasound image. This is because sound waves reflect differentially from different body tissues. This differential reflection is due to various factors, including the constituents and the density of the particular tissue being observed. The differentially reflected waves are then detected, usually with a transducer which can detect sound waves having a frequency of from 1 megahertz (MHZ) to 10 MHZ. The detected waves are integrated, quantitated and converted into an image of the tissue being studied.
Imaging vascularized tissue generally involves an analysis of difference in the acoustic properties between blood and tissues. Therefore, attempts have been made to develop contrast agents which serve to increase the acoustic difference between blood and surrounding tissues. This may also permit the measurement of blood flow, thereby improving detection of diseases associated with changes in blood flow. Contrast agents can serve to improve the quality and usefulness of images which are obtained with ultrasound. Certain exemplary contrast agents include, for example, suspensions of solid particles and emulsified liquid droplets.
The reflection of sound from a liquid-gas interface is extremely efficient. Accordingly, certain bubbles, including certain gas-filled bubbles, can be highly useful as contrast agents. The term "bubbles", as used herein, refers to vesicles which are generally characterized by the presence of one or more membranes or walls surrounding an internal void that is filled with a gas or precursor thereto. Exemplary bubbles include, for example, vesicles which are surrounded by monolayers and/or bilayers to form, for example, unilamellar, oligolamellar and/or multilamellar vesicles, such as liposomes, micelles and the like. As discussed more fully hereinafter, the effectiveness of bubbles as contrast agents depends upon various factors, including, for example, the size and/or elasticity of the bubble.
The effectiveness of bubbles as contrast agents depends upon various factors, including, for example, the size of the bubble. As known to the skilled artisan, the signal which is in the range of diagnostic ultrasound frequencies and which can be reflected off of a bubble is a fimction of the radius (r.sup.6) of the bubble (Rayleigh Scatterer). Thus, a bubble having a diameter of about 4 micrometer (.mu.m) possesses about 64 times the scattering ability of a bubble having a diameter of about 2 .mu.m. Thus, generally speaking, the larger the bubble, the greater the reflected signal.
However, bubble size is limited by the diameter of capillaries through which the bubbles must pass. Generally, contrast agents which comprise bubbles having a diameter of greater than about 10 .mu.m can be dangerous since microvessels may be occluded. Accordingly, it is preferred that greater than about 90% of the bubbles in a contrast agent have a diameter of less than about 10 .mu.m, with greater than about 95% being more preferred, and greater than about 98% being even more preferred. Mean bubble diameter is important also, and should be greater than about 1 .mu.m, with greater than about 2 .mu.m being preferred. The volume weighted mean diameter of the bubbles should be about 7 to about 20 .mu.m.
The viability of currently available ultrasound contrast agents and methods involving their use is highly dependent on a variety of factors, including the particular region being imaged. In certain circumstances, diagnostic artifacts may render a diagnostic image substantially unusable.
In addition to ultrasound, computed tomography (CT) is a valuable diagnostic imaging technique for studying various areas of the body. In CT, the radiodensity (electron density) of matter is measured and is expressed in terms of Hounsefield Units (HU). Hounsefield Units, named after the inventor of the first CT scanner, are an indication of the relative absorption of CT X-rays by matter, the absorption being directly proportional to the electron density of that matter. Water, for example, has a value of 0 HU, air a value of -1000 HU, and dense cortical bone a value of 1000 HU. Because of the similarity in the densities of various tissues in the body, however, it has been necessary to develop contrast agents which can be used to change the relative densities of different tissues. This has resulted in an overall improvement in the diagnostic efficacy of CT.
In the search for contrast agents for CT, researchers have generally sought to develop agents that will increase electron density in certain areas of a region of the body (positive contrast agents). Barium and iodine compounds, for example, have been developed for this purpose. For the gastrointestinal tract, barium sulfate is used extensively to increase the radiodensity of the bowel lumen on CT scans. lodinated water-soluble contrast media are also used to increase density within the gastrointestinal tract, but are not used as commonly as the barium compounds, primarily because the iodine preparations are more expensive than barium and are generally less effective in increasing radiodensity within this region of the body. The use of low density microspheres as CT contrast agents has also been reported. See, e.g., Unger, U.S. Pat. No. 5,205,290. As discussed above in connection with diagnostic methods for ultrasound, the viability of currently available CT contrast agents and methods involving their use for imaging the heart region is highly dependent on the flow of blood through the chambers of the heart relative to the flow of blood in the blood vessels of the heart tissue itself.
Magnetic resonance imaging (MRI) is another diagnostic imaging technique which may be used for producing cross-sectional images of the body in a variety of scanning planes such as, for example, axial, coronal, sagittal or orthogonal. MRI employs a magnetic field, radio frequency energy and magnetic field gradients to make images of the body. The contrast or signal intensity differences between tissues mainly reflect the T1 (longitudinal) and T2 (transverse) relaxation values and the proton density, which generally corresponds to the free water content, of the tissues. To change the signal intensity in a region of a patient by the use of a contrast medium, several possible approaches are available. For example, a contrast medium may be designed to change T1, T2, or the proton density.
Generally speaking, MRI requires the use of contrast agents. If MRI is performed without employing a contrast agent, differentiation of the tissue of interest from the surrounding tissues in the resulting image may be difficult. In the past, attention has focused primarily on paramagnetic contrast agents for MRI. Paramagnetic contrast agents involve materials which contain unpaired electrons. The unpaired electrons act as small magnets within the main magnetic field to increase the rate of longitudinal (T1) and transverse (T2) relaxation. Paramagnetic contrast agents typically comprise metal ions, for example, transition metal ions, which provide a source of unpaired electrons. However, these metal ions are also generally highly toxic. In an effort to decrease toxicity, the metal ions are typically chelated with ligands.
Metal oxides, most notably iron oxides, have also been employed as MRI contrast agents. While small particles of iron oxide, for example, particles having a diameter of less than about 20 nm, may have desirable paramagnetic relaxation properties, their predominant effect is through bulk susceptibility. Nitroxides are another class of MRI contrast agent which are also paramagnetic. These have relatively low relaxivity and are generally less effective than paramagnetic ions.
The existing MRI contrast agents suffer from a number of limitations. For example, increased image noise may be associated with certain contrast agents, including contrast agents involving chelated metals. This noise generally arises out of intrinsic peristaltic motions and motions from respiration or cardiovascular action. In addition, the signal intensity for contrast agents generally depends upon the concentration of the agent as well as the pulse sequence employed. Absorption of contrast agents can complicate interpretation of the images, particularly in the distal portion of the small intestine, unless sufficiently high concentrations of the paramagnetic species are used. See, e.g., Kornmesser et al., Magnetic Resonance Imaging, 6:124 (1988).
Other contrast agents may be less sensitive to variations in pulse sequence and may provide more consistent contrast. However, high concentrations of particulates, such as ferrites, can cause magnetic susceptibility artifacts which are particularly evident, for example, in the colon where the absorption of intestinal fluid occurs and the superparamagnetic material may be concentrated.
Toxicity is another problem which is generally associated with currently available contrast agents for MRI. For example, ferrites often cause symptoms of nausea after oral administration, as well as flatulence and a transient rise in serum iron. The gadolinium ion, which is complexed in Gd-DTPA, is highly toxic in free form. The various environments of the gastrointestinal tract, including increased acidity (lower pH) in the stomach and increased alkalinity (higher pH) in the intestines, may increase the likelihood of decoupling and separation of the free ion from the complex.
Blood flow may affect the quality of images obtained in MRI. For example, coronary vasodilators have been used in connection with thallium 201 (.sup.201 T1) in an effort to improve the visualization of viable myocardial tissue in nuclear medicine. Vasodilators can improve visualization by increasing blood flow to the myocardium which enables the .sup.201 T1 to be taken up more efficiently into viable myocardial cells. Coronary vasodilators have also been used in combination with Gd-DTPA to improve myocardial tissue imaging in MRI imaging. Although Gd-DTPA may be used as an indicator of blood flow, relaxation measurements (T1 and T2) may lack the necessary sensitivity to aid in the quantitative measurement of flow. In addition, MRI prior art contrast agents generally possess relatively low molecular weights which permits their diffusion through the vasculature. This may render the quantification of blood flow through the vasculature based on pharmacokinetics difficult.
Patients with renal vascular hypertension usually have a narrowing of one of the arteries to the kidneys, known as renal artery stenosis. Detection of renal vascular hypertension and distinguishing from the more common essential hypertension is critical, because renal artery hypertension does not respond to standard medical treatments administered for hypertension. Renal artery hypertension may be treated by angiotensin converting enzyme inhibitors or surgical management. Essential hypertension, in contrast, is usually treated with diuretics, beta or alpha blockers, afterload reducers, pre-load reducers and occasionally ganglionic blockers.
Diagnostic imaging may be used to detect the renal stenosis associated with renal hypertension. Imaging techniques used for imaging of the renal region include radionuclide scintigraphic methods and radionuclide nuclear medical methods. Captopril, an ACE inhibitor, has been used in combination with radiologic and radioscintigraphic procedures to detect stenosis in the renal artery. See, for example, Nally, et al., Sem. Nucl.
Med. XXII; 85-97 (1992), Itoh, et al., Clin. Nucl. Med. 18; 463-471 (1993) and Dondi, et al., J. Nucl. Med. 33; 2040-2044 (1992). Nuclear medicine, however, suffers from poor spatial resolution, high expense, and as discussed below, the undesirable necessity of employing radioactive materials. Angiography is preferred to nuclear methods for detection of renal hypertension, but it also is expensive and invasive. Attempts to use ultrasound as a diagnostic tool for renal hypertension so far have generally provided poor results. See for example, Postma, et al., Br. J. Radiol. 65;857-860 (1992) and Kliewer, et al., Radiol. 189;779-787 (1993).
Accordingly, new and/or improved diagnostic imaging methods, particularly for imaging the renal region, are needed. New and/or better diagnostic imaging methods which permit the quantification of blood flow in the renal region are also needed. The present invention is directed to these, as well as other, important ends.